As a switching power supply device for various kinds of electronic instruments, a resonant converter is known. The resonant converter is configured by connecting a primary winding of an isolation transformer to a direct current voltage source via a capacitor. A series resonant circuit is formed of the leakage inductance of the isolation transformer and the capacitor. The resonant converter controls a current flowing through the series resonant circuit using first and second switching elements which are complementarily on/off driven, and obtains a stepped-up/down direct current voltage from the side of secondary windings of the isolation transformer.
A soft switching technology in this kind of switching power supply device is proposed in, for example, U.S. Pat. Nos. 5,886,884 and 7,391,194. The soft switching technology is such as to significantly reduce loss in the switching elements by turning on the switching elements when a voltage applied to the switching elements is zero (0) or when a current flowing through the inductance is zero (0). Also, as the switching power supply device, a multi-oscillated current resonant converter which causes the series resonant circuit to perform a current resonance operation is also proposed. The multi-oscillated current resonant converter, as well as causing the first switching element to perform a separately excited oscillation operation with a drive signal pulse-width controlled in accordance with an output voltage, causes the second switching element to perform a self-oscillation operation utilizing a voltage generated in an auxiliary winding of the isolation transformer.
This kind of resonance type switching power supply device includes a series resonant circuit which, by connecting a primary winding P1 of an isolation transformer T to a direct current voltage source B via a capacitor C, as shown in, for example, FIG. 5, is formed of the leakage inductance of the isolation transformer T and the capacitor C. A first switching element Q1 connected in series to the primary winding P1 of the isolation transformer T, by being driven by a drive control circuit A which performs a separately excited oscillation operation, applies a direct current input voltage Vin from the direct current voltage source B to the series resonant circuit. The drive control circuit A is formed of, for example, a power supply IC (Integrated Circuit). Also, a second switching element Q2 connected in parallel to the series resonant circuit, by being on-driven by the drive control circuit A when the first switching element Q1 is turned off, forms a resonant current path of the series resonant circuit. The first and second switching elements Q1 and Q2 are each formed of, for example, a high-voltage n-type MOS-FET (Metal-Oxide-Semiconductor Field-Effect Transistor).
Power generated in secondary windings S1 and S2 of the isolation transformer T is rectified and smoothed via an output circuit formed of diodes D1 and D2 and an output capacitor Cout, and supplied to an unshown load as an output voltage Vout. A resonance type power conversion device main body is constructed by these circuit portions. Also, the output voltage Vout obtained in the output circuit, specifically, the deviation between the output voltage Vout and an output voltage set value, is detected by an output voltage detection circuit VS and fed back to the drive control circuit A as an FB voltage via a photocoupler PC. The FB voltage fed back to the drive control circuit A serves to pulse-width modulate an output control signal which on/off drives the first and second switching elements Q1 and Q2, thereby stabilizing the output voltage Vout. Direct current power supplied from the direct current voltage source B, after being filtered via an input capacitor Cin, is supplied to the switching power supply device as the input voltage Vin.
Herein, the drive control circuit A, as an outline configuration thereof is shown in FIG. 6, is configured mainly of an output control circuit 2, a winding detection circuit 3, and a drive signal generation circuit 4. The output control circuit 2 is formed of a PWM (Pulse-Width Modulation) control circuit which generates, as a PWM signal, an output control signal with a pulse width corresponding to a feedback signal FB fed back from the output voltage detection circuit VS. Also, the winding detection circuit 3 determines the polarity of a voltage generated in a tertiary winding P3 of the isolation transformer T, and outputs a winding detection signal VW. The drive signal generation circuit 4, in accordance with the winding detection signal VW and output control signal, generates a pulse-width controlled drive signal of the first switching element Q1.
Reference numeral 5 in FIG. 6 is a drive amplifier acting as a drive circuit which drives the first switching element Q1 upon receiving the drive signal output by the drive signal generation circuit 4. Also, reference numeral 6 is an internal power supply circuit which, upon receiving a drive voltage VCC applied to the drive control circuit A, generates an internal voltage VDD necessary for the output control circuit 2, winding detection circuit 3, and drive signal generation circuit 4 to operate.
A brief description will be given of an operation of the multi-oscillated current resonant converter which is the switching power supply device of the heretofore described configuration. In the multi-oscillated current resonant converter, when the second switching element Q2 is in an off-state, a current flows through the series resonant circuit by turning on the first switching element Q1. In this condition, when the first switching element Q1 is turned off, an unshown parasitic capacitance of the first switching element Q1 is charged by a current flowing through the inductance of the series resonant circuit. At the same time, an unshown parasitic capacitance of the second switching element Q2 is discharged by the current.
Further, when a voltage charged in the parasitic capacitance of the first switching element Q1 reaches the direct current input voltage Vin, the second switching element Q2 is turned on, thereby realizing the zero-voltage switching of the second switching element Q2. As a result of turning on the second switching element Q2, power energy accumulated in the capacitor C flows via the second switching element Q2. Consequently, the current flowing through the inductance of the series resonant circuit is inverted.
Subsequently, when the second switching element Q2 is turned off, the parasitic capacitance of the second switching element Q2 is charged by the current inverted in the way heretofore described. At the same time, the parasitic capacitance of the first switching element Q1 is discharged by the inverted current. Further, it is detected, from an inversion of the polarity of the voltage generated in the tertiary winding P3, that the voltage charged in the parasitic capacitance of the second switching element Q2 reaches a zero (0) voltage. The first switching element Q1 is turned on at this detection timing, thereby realizing the zero-voltage switching of the first switching element Q1. By the first switching element Q1 being turned on, the current of the series resonant circuit is inverted and flows again via the first switching element Q1.